The present invention relates to a process for the manufacture of steel balls, particularly balls of small and medium dimensions for rolling element bearings, by means of which the balls can be produced by employing a significantly smaller quantity of energy than that necessary with conventional processes, and utilising apparatus and installations which are very much simpler and easier to operate than those required in known processes.
As is known, the production of steel balls for rolling element bearings involves a process which comprises very many phases both of a metallurgical character, for the preparation of blanks (ingots) suitable for subsequent working, and by hot mechanical working to obtain steel rod from the ingots, and by cold mechanical working to convert the steel rod into balls.
The molten steel is first cast into ingots from which, by means of hot rolling, there are successively obtained blooms, billet, bars and rods. From this latter, by means of cutting and pressing on presses of a special type, blanks delimited by two caps having spherical surfaces, separated by an annular projection contained in a diametral plane are obtained. These blanks are then worked by means of an operation to remove shavings or chips on suitable shaping machines to remove the said annular projections from them, treated in rotating drums to obtain more regular surfaces, and then ground first for roughing out and then finishing on special grinding machines, for the purpose of obtaining balls with good tolerances in their shape and dimensions. Subsequently the balls are subjected to heat treatments for hardening and tempering and to further working by way of grinding for finishing and polishing.
It is apparent that the phases of the process described require a large quanity of energy both because the form which is imparted to the blanks in the successive working phases is obtained by removing material from the blank obtained in the preceding phase or by modifying its shape by the application of pressures and/or high temperatures, and because some of these phases must be performed hot or require significant forces for causing plastic deformation or for removing material.
Now, The apparatus and installations required to undertake all these phases are complex and expensive and require particularly skilled operators for operating them. Consequently the cost of production of balls using the described process is rather high. Moreover, in the balls thus obtained there is a certain anisotropy in the material, especially in the outermost layers of each ball, due to the phase of cold plastic deformation by which the cylindrical blank is converted to an approximately spherical form. This anisotropy is manifested in the form of oriented fibres deriving from the orientation of the fibres in the rod, which remain in the spherical form and which results in the formation of poles. This anisotropy of the structure gives rise to an asymmetric distribution in the Hertzian stresses when a load is applied to the ball.